1. Field of the Invention
The present invention relates to a nuclear magnetic-resonance angiographic imaging for taking an image of a flowing stream section such as a blood vessel of a patient by utilizing a nuclear magnetic resonance imaging technique.
2. Description of the Background Art
As a conventional method for imaging a flowing stream section such as a blood vessel of a patient by using a nuclear magnetic resonance, a so called bolus tracking method is known.
A typical pulse sequence used in this bolus tracking method is shown in FIG. 1, where RF indicates the RF pulses, G.sub.S indicates the slicing gradient magnetic field, G.sub.E indicates an encoding gradient magnetic field, and G.sub.R indicates a reading gradient magnetic field, while the nuclear magnetic resonance images obtained by using this pulse sequence are shown in FIGS. 2(A), 2(B), and 2(C).
In this bolus tracking method, a pre-saturation section L.sub.0 within an imaging region 9 formed over a blood stream in a blood vessel 8 is excited first by a pre-saturation excitation pulse P.sub.0 for pre-saturating this pre-saturation section L.sub.0 such that the blood stream located at this pre-saturation section L.sub.0 at that point will not produce the nuclear magnetic resonance signals subsequently. Then, a large slicing gradient magnetic field G.sub.S0 which is larger than the usual slicing gradient magnetic field G.sub.S is applied to the imaging region 9 in order to disperse and thereby delete the transverse components of spins produced by the excitation due to the application of the pre-saturation excitation pulse P.sub.0. Next, the imaging region 9 is excited again by the application of an imaging excitation pulse Pi. As a result, the nuclear magnetic resonance signals S.sub.1 are produced at a time t.sub.1 (an echo time) after the application of the pre-saturation excitation pulse P.sub.0, which are collected by using the encoding gradient magnetic field G.sub.E in 64 to 256 different field strengths and the reading gradient magnetic field G.sub.R.
By this procedure, it is possible to obtain an image D.sub.1 shown in FIG. 2(A) which shows a pre-saturated pattern PT.sub.1 due to the blood stream located at the pre-saturation section L.sub.0 at a time of the application of the pre-saturation excitation pulse P.sub.0 which has subsequently moved to a position of the pre-saturated pattern PT.sub.1 during the time t.sub.1. In FIG. 2(A), the pre-saturation section L.sub.0 and the pre-saturated pattern PT.sub.1 are shadowed because they appear as black areas on the resulting nuclear magnetic resonance image in contrast to the remaining area, as no nuclear magnetic resonance signal can be received from these pre-saturated sections.
Now, by using the similar procedure with the longer echo time t.sub.2, an image D.sub.2 shown in FIG. 2(B) showing a pre-saturated pattern PT.sub.2 at a position further ahead in the blood vessel 8, and by using the similar procedure with the still longer echo time t.sub.3, an image D.sub.3 shown in FIG. 2(C) showing a pre-saturated pattern PT.sub.3 at a position still further ahead in the blood vessel 8 can be obtained.
Then, by displaying these images D.sub.1, D.sub.2, and D.sub.3 sequentially, it is possible to realize the cine display in which the black area of the pre-saturated portion moves from the pre-saturation section L.sub.0 to the pre-saturated patterns PT.sub.1, PT.sub.2, and PT.sub.3 sequentially, such that the actual motion of the blood stream in the blood vessel 8 can be visually represented.
However, in order to realize such a cine display by using the conventional nuclear magnetic resonance angiographic imaging, a considerably lengthy imaging operation was necessary, and the reduction of this imaging operation time has been an outstanding problem in the nuclear magnetic resonance angiographic imaging.